Exhaust-gas streams often contain combustible pollutants in gas and/or particulate form and/or NOx, i.e. nitrogen oxides. These are often produced at a relatively low temperature, and consequently heating to the temperature required for thermal or catalytic conversion of the pollutants is required. Heating of this type always requires high quantities of energy if the exhaust-gas streams are large and the pollutant concentrations are so low that combustion of the pollutants is unable to make any significant energy contributions. This applies in particular to the exhaust gas from internal combustion engines, where, during a cold start or when the engine load is low, the exhaust gases are produced at a temperature which is too low for the known catalytic exhaust-gas purification methods. This applies in particular to diesel engines or to spark-ignition engines which are operated in clean-burn mode.
According to the general prior art, it is preferable for what are known as autothermal exhaust-gas converters, in which the pollutants are burnt, with or without additional supply of heat, at a catalyst or in a combustion chamber and the hot exhaust gas is used to heat the cold feed, to be used for the exhaust-gas purification to remove gaseous, combustible constituents.
Furthermore, the engine exhaust-gas purification requires apparatuses for retaining and/or converting nitrogen oxides, preferably in lean-burn engines, and apparatuses for retaining and converting particulates, in particular soot particulates, preferably in diesel engines. The engine exhaust-gas train also includes apparatuses for muffling and if appropriate also for discharging heat, in order to limit the exhaust-gas temperature and/or to heat the passenger compartment.
According to the prior art as it stands at present, the various functions which have been mentioned above are realized by additive elements, such as primary catalyst, main catalyst, diesel soot filter, exhaust-gas cooler, muffler, and the exhaust-gas conditions which are required for these elements are primarily established by interventions in the electronic engine control. This entails high costs for the exhaust-gas train, problems with installation space and extensive linking to the engine control. This prevents the engine control from being optimized exclusively for optimum, low-consumption driving operations.
It is an object of the invention to provide a method showing how the different exhaust-gas treatment steps required for spark-ignition lean-burn engines and diesel engines can be combined in an autonomous and compact exhaust-gas treatment unit which is independent of the engine control, and which components and apparatuses can expediently be used to do so. For this purpose, in the text which follows, first of all the prior art relating to the individual components and the proposals which have been made hitherto for integrated engine exhaust-gas purification will be summarized. The invention will then be explained and substantiated with reference to the prior art.
Prior Art: Exhaust-Gas Heat Exchangers
What is known as autothermal reaction has long belonged to the prior art for the conversion of combustible exhaust-gas constituents in cold exhaust gases. In this procedure, the hot discharge from the reaction zone is used to preheat the cold feed. A separate supply of heat in the hot part of the exhaust-gas reactor is generally provided for starting up the cold reactor and to compensate for heat losses. Either recuperative/indirect or regenerative/direct heat exchange with a fixed heat accumulator is used for the heat exchange between the hot exhaust gas and the cold feed.
The regenerative heat exchange can be carried out very efficiently. It is particularly suitable for medium and high volumetric flows and requires periodic switching of the exhaust-gas streams. By contrast, the recuperative operation takes place continuously and is therefore also eminently suitable for low and medium exhaust-gas streams. In practice, by way of example, tube-bundle heat exchangers are used for this purpose; these are complex to produce in particular if a high quality of return heat transfer is to be achieved. Parallel-passage heat exchangers in the form of plate-type heat exchangers are more suitable. These can also be combined with a reaction unit for the catalytic converters.
With regard to parallel-passage heat exchanger reactors of this type, reference is made to EP 0 638 140 B1, U.S. Pat. No. 6,207,116 B1, EP 1 016 777 A2, EP 0 855 653 B1 and DE 100 40 209 A1, which describe passage arrangements in which a catalyst which is suitable for exhaust-gas purification may be arranged either on the wall or on additional structures in the passages.
Plate-type heat exchangers are generally designed with elastic seals between the individual plates. The required sealing pressure is achieved by means of tie-rods between the solid end plates. Designs of this type are unsuitable for catalytic or thermal exhaust-gas purification, on account of the high temperatures that occur and for weight reasons.
Continuously welded or soldered plate-type heat exchanger arrangements have been disclosed, for example, for fuel cell systems with integrated fuel reforming. They avoid the need for elastic seals and solid endplates, but the large number of welded or soldered seams are expensive and susceptible to faults. Moreover, the multiple flow diversion in conventional plate-type heat exchanger arrangements leads to a pressure loss which is unacceptable for exhaust-gas purification and possibly also to uncontrolled deposition and blockage caused by exhaust-gas constituents in particulate form.
The parallel-passage concepts described in U.S. Pat. No. 6,207,116 B1 and EP 0 855 653 B1 are distinguished by simple separation of the passages by a concertina-like folding and a simple flow guidance. However, in these concepts, the design of the baseplates and the seal between the passages and the outer housing required there has proven difficult in design and manufacturing technology terms. Moreover, the flow diversion at the entry and exit leads to an increased pressure loss. Although U.S. Pat. No. 6,207,116 B1 describes an embodiment which avoids the base sealing by flow division, in this arrangement it is necessary to provide two of all the installation elements. In addition, the profiling of the partition walls which is required there leads to an additional pressure loss. As our own tests have shown, the lateral inlet and the profiling of the folding can also lead to increased accumulation of soot and premature blockage. This situation is exacerbated if this blockage occurs in the cold feed region and cannot be eliminated by soot burn-off initiated by the supply of heat in the diversion.
For these multifarious reasons, the proposed plate-type heat exchanger concepts have not hitherto gained widespread acceptance in catalytic or thermal exhaust-gas purification practice. An additional factor for the automotive industry was the additional weight and pressure loss, and consequently these solutions have not thus far entered any form of series production.
Prior Art: Heat Source
Even when using an efficient exhaust-gas heat exchanger, a heat source, which heats the region of the exhaust-gas converter which is crucial for the pollutant conversion to a temperature required for the conversion, is required at least to start up the cold exhaust-gas converter. Hitherto, in stationary autothermal exhaust-gas purification installations, this heating has been effected either electrically or by an auxiliary burner, the hot exhaust gas from which is fed to the hot side of the exhaust-gas heat exchanger.
On account of the return heat transfer, the power of the additional heating in autothermal exhaust-gas purification installations can be greatly reduced or switched off, even with a cold feed, as soon as the reaction region of the converter has reached its operating temperature.
Autothermal reaction in combination with a supply of heat has also been proposed numerous times for the purification of the exhaust gas from automobiles. For example, U.S. Pat. No. 6,207,116 B1, EP 1 016 777 A2, WO 02/29218 A1, DE 101 05 185 A1 and US 2003 0074888 A1 have disclosed a heat source in the hot part of the exhaust-gas heat exchanger, which may be designed in the form of an electrical heating means, a fuel burner or a hot-gas feed. The heat source in the hot part of an exhaust-gas heat exchanger can therefore be regarded as an integral part of any autothermal exhaust-gas treatment unit.
In series-production motor vehicles, however, the catalyst has hitherto been heated exclusively by means of the hot engine exhaust gas. For this purpose, a hot exhaust gas, which quickly heats up the catalyst, is generated in the exhaust pipes upstream of the exhaust-gas converter, for example by means of the engine control and/or by postinjection and ignition of fuel. Electrically heatable primary catalysts are also used. A fuel-rich exhaust gas is ignited at the primary catalysts after an engine cold start. The heat which is released in the process subsequently heats the main catalyst.
A drawback of these concepts is the fact that the heat which is introduced or released does not, as in the autothermal concepts, remain largely in the hot part of the converter, but rather is discharged, resulting in increased energy consumption. This applies in particular to low-load operation of diesel and spark-ignition lean-burn engines, in which the exhaust gas is generally at a temperature which is too low for pollutant conversion.
Prior Art: NOx Conversion
A common feature of all the methods known for conversion of NOx into molecular nitrogen is that they only produce usable results at relatively high temperatures (above 250° C.). In many operating modes, the exhaust-gas temperatures of spark-ignition lean-burn engines, and in particular diesel engines, are often lower than these levels, consequently hitherto the engine control has had to intervene if the temperature range required for pollutant conversion is to be reached. This leads to increased fuel consumption.
In the concepts which have been developed thus far, the reducing agent which is required for most NOx conversion methods is either released from an auxiliary substance which is additionally carried in the vehicle, such as for example urea, or is generated by engine measures, such as for example postinjection, in the engine or in the hot exhaust pipe, so that a reducing exhaust gas is temporarily formed. The urea variant requires additional outlay for refueling, storage and conversion. Generation by postinjection leads to an increased fuel consumption, since the excess oxygen of all the exhaust gas has to be consumed by the postinjection.
In what is known as the storage catalyst concept, the reducing agent CO and H2 has hitherto generally been generated by the engine control briefly adjusting engine operation in such a way as to form a rich (reducing) exhaust gas with a high CO and H2 content. This likewise leads to increased fuel consumption and may therefore disadvantageously restrict the driving dynamics. Moreover, in the case of sulfur-containing fuel, what is known as sulfur regeneration is also required at relatively spaced-apart intervals in NOx storage catalysts; during this sulfur regeneration, the catalyst has to be heated to temperatures of from 650 to 750° C. under reducing conditions. This increasing of the temperature has also hitherto been affected exclusively by way of the engine control.
Even in the exhaust-gas purification concepts with integrated heat exchange and dedicated heat source which have been disclosed hitherto, the reducing conditions with a high CO and H2 content which are required for the regeneration of the storage catalyst have always been set by means of the engine control. For example, DE 101 05 185 A1 and US 2003/0074888 A1 have described exhaust-gas treatment units with storage catalyst and integrated heat source, with the heat source being used exclusively to set the required temperature range.
Prior Art: Particulate or Soot Filtering and Filter Regeneration
The standard method for separating off particulate constituents is filtering. Since the filter becomes blocked as a result, it has to be regenerated at periodic intervals or continuously. If the particulates are combustible, an obvious procedure is to burn off the filter loading. This can take place either at periodic intervals, by the filter temperature for this purpose briefly being increased to above the ignition temperature of the particulate, or continuously, by the filter always being operated at a temperature which is sufficiently high for the particulates which are deposited to burn off more quickly than they are delivered.
Both options are used for diesel particulate filtering. For this purpose, the deposition usually takes place on porous ceramic monoliths, in which the untreated-gas passages and purified-gas passages are arranged in checkerboard fashion in the monolith. The purified-gas passages are closed off at the filter inlet and the untreated-gas passages are closed off at the filter outlet, so that the filtering takes place through the porous passage walls.
Continuous filter burn-off is possible substantially without problems if the untreated-gas temperature is sufficiently high. With standard diesel exhaust gas, the minimum temperature required is of the order of magnitude of 600° C. The required burn-off temperature can be reduced to below 400° C. by the addition of additives to the fuel or by prior catalytic oxidation of NO and utilization of the NO2 which is formed for soot burn-off.
However, since the exhaust gas is significantly colder in particular in the low-load range, considerable additional energy would be required to raise the exhaust gas to these temperatures without return heat transfer.
Therefore, many diesel filter concepts provide for the exhaust gas to be heated to the ignition temperature only at periodic intervals when a sufficient filter loading has built up. These concepts function satisfactorily whenever the regeneration is triggered at an optimum filter loading level. If the filter loading level is too low, there is a risk of incomplete ignition and incomplete burn-off, with the result that the loading level is too high during the next regeneration. If the filter loading level is too high, there is a risk of the development of a burn-off front with an uncontrollable maximum temperature, at which the filter and, in certain circumstances, even the entire vehicle is destroyed.
As our own tests have shown, a burn-off front with an uncontrollable maximum temperature rise can occur whenever the incoming flow into the filter is in the same direction as the outgoing flow and the burn-off is ignited by increasing the temperature of the incoming gas. On account of the inherent risks of burn-off, diesel soot filters are currently only used with extensive precautionary measures in series-production vehicles.
In addition to the abovementioned ceramic filters, numerous filter variants have been described in which surface deposition, if appropriate assisted by electrostatic charging and agglomeration of the particulates, is to be effected.
The prior art had also disclosed the combination of a particulate filter with a countercurrent heat exchanger (Opferkuch, Gaiser, Eigenberger: “Enffernung oxidierbarer Aerosole aus Abluftströmen” [Removal of oxidizable aerosols from waste air streams], FZKA-BWPLUS 8 (1998), http://www.bwplus.fzk.de) and “Gegenstromreaktor zur Entfernung oxidierbarer Aerosole aus Abluftströmen” [Countercurrent reactor for removing oxidizable aerosols from waste air streams] BW-Plus-Research project No. 397007, concluding report February 2001 (http://bwplus.fzk.de/berichte/SBer/PEF397007Ber.pdf). In this case, the particulate filter is arranged in the hot part of an exhaust-gas heat exchanger and is held, by a heating apparatus upstream of the filter, at a temperature which is sufficiently high for the particulates which are separated out to be completely burnt on account of the increased residence time. The method described can also be used to periodically regenerate the filter by periodically increasing the filter entry temperature. However, the incoming flow is in the same direction as the outgoing flow, with the result that the above-described uncontrollable temperature rise during the periodic soot burn-off can occur. WO 02/29218 A1 also describes the integration of a filter element in the hot part of a countercurrent heat exchanger. It emerges from the description that this is evidently to involve surface deposition on the profiled walls of the flow passages, which may be boosted by a filter mat arranged between the walls. As has already been explained above under the heading “Prior Art: Exhaust-gas heat exchangers”, this design can lead to rapid blockages in the cold part during diesel particulate filtering.
For these multifarious reasons, the automotive industry is still heavily engaged in looking for technologically simple solutions which allow successful separation of soot and reliable soot filter regeneration with a low pressure loss and a low additional energy consumption.
Prior Art: Integration of Automobile Exhaust-Gas Components
The various components used for the exhaust-gas treatment of engine exhaust gas by now represent a significant part of the costs of the drive train. In addition to the exhaust-gas catalyst, the NOx catalyst and the diesel soot filter, these components also include the exhaust-gas muffler and possibly also devices for heating the passenger compartment or stationary heating.
If the operation of the automobile exhaust-gas converter, as is currently standard, is influenced by the engine control, it has under certain circumstances proven necessary to divide the catalyst into a number of units and for these units to be arranged close to and away from the engine. This contributes to further increases in costs and possibly also to an increase in weight.
By contrast, combining all the required units to form a single exhaust-gas module and controlling it autonomously so as to be independent of the engine control, would have cost, functional and possibly also weight advantages.
However, this type of combination has hitherto appeared only partially possible. For example, as described above, both the integration of automobile exhaust-gas catalysts, of NOx storage catalysts or of soot filters in the correspondingly temperature-controlled parts of an exhaust-gas heat exchanger has been proposed, with the additional energy required being supplied electrically or by means of a fuel burner. However, in both DE 101 05 185 A1 and US 2003/0074888 A1, it is assumed that the reducing and CO- and/or H2-containing atmosphere required for the regeneration of the NOx storage catalyst has to be set by altering the engine control with postinjection in the cylinder or the adjoining exhaust pipe. The descriptions of US 2003/0074888 A1 and WO 02/29218 A1 likewise assume that, as has hitherto been the case, a plurality of separate exhaust-gas treatment components, such as primary catalyst and main catalyst, are present and these components cannot be operated independently of the engine control.